Threaded connection especially for radically plastically expandable conduit

ABSTRACT

A threaded connection includes a male member having threads thereon defining a load flank lead and a stab flank lead. The threaded connection also includes a female member having threads thereon adapted to mate with threads on the male member, wherein the threads on the female end define a load flank lead and a stab flank lead, wherein at least one of the load flank lead and the stab flank lead on at least one of the female member and the male member change at a predetermined rate beginning at a selected distance from either one or both ends of the threads, wherein the stab flank lead and the load flank lead are different from each other over at least part of the thread length.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application claiming the benefitof U.S. patent application Ser. No. 10/126,918 entitled “ThreadedConnection Especially for Radially Plastically Expandable Conduit,”filed on Apr. 19, 2002.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to threaded connections used for couplesegments of pipe. More specifically, the invention relates to structuresfor threaded connections that may be used particularly in radially,plastically expandable tubes or pipes.

2. Background Art

Threaded tubular connections are used for joining segments of conduitsor pipes end-to-end to form a continuous conduit or pipe fortransporting fluid under pressure. Oilfield tubular goods, such ascasing, generally use such threaded connections for connecting adjacentsections of conduit or pipe. Examples of such threaded connectionsdesigned for use on oilfield tubular goods are disclosed in U.S. Pat.Nos. 2,239,942; 2,992,019; 3,359,013; RE 30,647; and RE 34,467.

In U.S. Pat. No. RE 30,647 issued to Blose, a particular thread form orstructure is disclosed for a tubular connection that provides anunusually strong joint while controlling the stress and strain inconnected “pin” (male thread) and “box” (female thread) members towithin acceptable levels. The pin member has at least one generallydovetail-shaped external thread whose width increases in one directionalong the pin, while the box member has at least one matching generallydovetail-shaped internal thread whose width increases in the otherdirection. The mating set of dovetail-shaped threads provide awedge-like engagement of opposing pin and box thread flanks that limitthe extent of relative rotation between the pin and box members, anddefine a forcible make-up condition that completes the connection. Inthis thread structure, the angles of the flank shoulder, as well as thethread width, can be used to control the stress and strain preloadconditions induced in the pin and box members for a given make-uptorque. Thus, by tailoring the thread structure to a particularapplication or use, the tubular connection or joint is limited only bythe properties of the materials selected.

A prior art tubular threaded connection includes a pin member and a boxmember. The box member has a tapered, internal, in many cases generallydovetail-shaped thread structure formed thereon which is adapted forengaging complementary tapered, external, thread structure formed on thepin member to mechanically secure the box and pin members in areleasable manner.

Internal thread on the box member has stab flanks, load flanks, roots,and crests. The internal thread increases in width progressively at auniform rate in one direction over substantially the entire helicallength of thread. External thread on the pin member has stab flanks,load flanks, roots, and crests. The external thread increases in widthprogressively at a uniform rate in the other direction, oversubstantially the entire length of the external thread. The oppositelyincreasing thread widths, and the taper of threads cause thecomplementary roots and crests of the respective threads and to moveinto engagement during make-up of the connection in conjunction with themoving of complementary stab and load flanks into engagement uponmake-up of the connection.

The pin member or the box member defines the longitudinal axis of themade-up connection. The roots and crests of the box and pin members insome cases are flat and parallel to the longitudinal axis of theconnection and have sufficient width to prevent any permanentdeformation of the threads when the connection is made up.

An important part of any connection is a seal for keeping the conduitfluid pressure-tight at the connections. Typically connections will bedesigned to include metal-to-metal seals therein. Metal-to-metal sealshave the advantage of not requiring gaskets or other additional sealingdevices, which would typically have to be replaced periodically as theconnections are coupled and uncoupled. Metal seals are created whencontact pressure between two metal surfaces exceeds the fluid pressureto be sealed. Typically the contact pressures are created during make upof the connection. Some types of metal to metal seal are energized byinternally pressurizing the conduit.

More recently, oilfield tubular goods have been developed which can beplastically radially expanded from their initial diameters after beinginstalled for the intended application. See for example, R. D. Mack etal, How in situ expansion affects casing and tubing properties, WorldOil, July 1999, Gulf Publishing Co., Houston, Tex., for a description ofradially expandable oilfield tubular goods. Radially expandable tubulargoods have particular application as casing in oil and gas producingwells. It has been difficult to seal radially expandable tubularconnections using metal-to-metal seals known in the art.

It has also been determined that conventional threaded connections,including the previously described variable width threads, undergo largechanges in distribution of stresses when such couplings are radially,plastically expanded. It is desirable to have a threaded connectionwhich can maintain strength and sealing ability even after plasticradial expansion. It has also been determined that threaded connectionssuch as the previously described variable thread width connections mayhave uneven stress distribution when the connection undergoessubstantial compressive or tensile stress. It is therefore desirable tohave a threaded connection which is better able to resist tensile andcompressive stresses.

SUMMARY OF INVENTION

One aspect of the invention is a threaded connection for a radially,plastically expandable conduit. The threaded connection includes a malemember having threads on it which define a load flank lead, a stab flanklead and a nominal lead. A female member has threads on it that areadapted to mate with the threads on the male member. The threads on thefemale member also define a load flank lead, a stab flank lead and anominal lead. At least one of the load flank lead and the stab flanklead, on at least one of the female member and the male member, arevaried at a predetermined rate beginning at a selected distance from anend of the threads. The load lead and the stab lead are different fromeach other over at least part of the thread length.

In one embodiment, the load flank lead, the nominal lead and the stabflank lead are each changed at a corresponding rate at the selecteddistance from at least one corresponding thread end on the male memberand the female member.

In one embodiment, the stab flank lead is decreased at a nose of themale member and the stab flank lead is decreased at the thread base ofthe female member. The stab flank lead is substantially equal to thenominal lead at the nose of the male member and at the thread base ofthe female member. In one specific variation of this embodiment, theload flank lead is increased at a thread base of the male member and theload flank lead is increased at an open end of the female member. Theload flank lead is substantially equal to the nominal lead at the baseof the male member and at the open end of the female member.

In some embodiments, the stab lead is decreased and the load lead isincreased near a first engaged thread on both the male member and thefemale member, and the load lead is decreased and the stab lead isincreased near a last engaged thread on both the female member and themale member.

In some embodiments, near a first engaged thread on both the femalemember and the male member, the load lead and the stab lead areincreased.

In some embodiments, on the male member the load lead is reduced near alast engaged thread thereof, and on the female member, the stab lead isincreased near a last engaged thread thereof.

In some embodiments, the change in lead is linear. In some embodiments,the selected distance is about two threads.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A shows a cut away view of one side of a female end of a threadedconnection.

FIG. 1B shows a cut away view of a male end and a female end of one sideof a threaded connection, as the ends are joined or “made up”.

FIG. 1C shows a cut away view of a male end of one side of a threadedconnection.

FIGS. 2 through 20 are graphs of stab lead, load lead and nominal leadfor exemplary embodiments of a threaded connection according to theinvention.

FIG. 21 shows an alternative thread form that can be used with variousembodiments of the invention.

FIG. 22 shows an alternative thread form that can be used with variousembodiments of the invention.

FIG. 23 shows an embodiment of the invention which may have increasedcapacity to withstand tensile stress.

FIG. 24 shows an embodiment of the invention which may have increasedcapacity to withstand compressive stress.

FIGS. 25A and 25B show an embodiment of the invention which may haveincreased capacity to withstand torque and fatigue.

DETAILED DESCRIPTION

In its most general terms, the invention is a threaded connectionparticularly suited for use in radially, plastically expandable conduit,or in some embodiments may be particularly suited for use inapplications where the connection is expected to undergo substantialtensile or compressive stress. Embodiments of the invention include atype of thread known by the trademark “Wedge Thread.” “Wedge Thread” isa trademark of Hydril Company, Houston, Tex., the assignee of thepresent invention, for threaded connections having a load flank leadwhich is different from the stab flank lead.

FIG. 1B shows a cut away view of a male member (pin) 3 coupled to afemale member (box) 2 of a threaded connection 1 as the pin 3 and box 2are joined. Only one side of the connection 1 (with respect to thecenterline or longitudinal axis of the connection) is shown for clarity,the other side would thus be a mirror image of the side shown in FIG.1B. The pin 3 terminates at a pin nose 18, and begins at a pin threadbase 22. A distance 24 between the pin nose 18 and the thread base 22 isknown as the thread length 24. Between the nose 18 and the thread base22 are threads 26. In the example shown in FIG. 1B, the threads 26 aregenerally dovetail shaped. As will be further explained, other types ofthreads are also suitable for use in a threaded connection according tothe invention.

The box 2 terminates in an open box end 16, and starts at a box threadbase 20. Between the box thread base 20 and the box end 16, the innersurface of the box 2 includes threads 26 which are adapted to mate withthe threads 26 on the pin 3.

A characteristic of threads used in various embodiments of the inventionis that the “lead” of the threads is generally different on one threadflank than on the other thread flank. FIG. 1A shows the section of thebox 2 of FIG. 1B in more detail. The threads each include a stab flank12A and a load flank 14A. A distance, along the longitudinal axis of theconnection, between each successive stab flank 12A, shown at 12 isreferred to as the stab flank lead, or stab lead. The distance betweeneach successive stab flank 12A is generally defined as the axial span(distance along the longitudinal axis of the pin or box) betweencorresponding points on successive threads at the same angular positionaround the circumference of the box 2. A similar definition applies tothe load lead 14. In threaded connections according to the invention,the stab lead 12 is different than the load lead 14 by a selected amountknown as the wedge ratio. As will be further explained, the wedge ratiois maintained at the selected amount up to a selected position fromeither or both ends of the threads in various embodiments of theinvention. A “nominal” or “mid-lead” is a value mid way between the stablead 12 and the load lead 14.

Similar definitions of stab lead 12 and load lead 14 apply to thecorresponding load flanks and stab flanks on the pin threads. The loadand stab leads on the pin are shown in more detail in FIG. 1C at 12 and14, respectively for the stab and load leads. Typically, the threadedconnection (1 in FIG. 1B) will be tapered. Tapered in this descriptionmeans the diameter of the inner surface of the box 2 and the diameter ofthe outer surface of the pin 3 will change with respect to the axialposition along the thread length (24 in FIG. 1B). The generalconstruction and operation of the threaded connection 1 shown in FIG. 1Bis known in the art, and is described, for example, in U.S. Pat. No. RE30,647 issued to Blose, and is also generally described in theBackground section herein.

An example of a typical differential lead thread known in the art for athreaded connection is shown graphically in FIG. 2. The graph in FIG. 2shows lead on the stab flanks of the thread at 12, the nominal ormid-lead at 10, and the load flank lead at 14. The coordinate axis ofthe graph indicates the engaged thread number of the member (pin orbox), starting at the open end of the member and ending at the threadbase end of that member. For example, thread number 1 represents thefirst engaged thread proximate the nose end of the pin (male member—noseshown at 18 in FIG. 1B) or the first engaged thread proximate the openend of the box (female member—open end shown at 16 in FIG. 1B). Threadnumber 10 would thus be near the thread base of the pin (22 in FIG. 1B),or near the thread base end of the box (20 in FIG. 1B). Differentiallead threads known in the art include a stab lead 12 which issubstantially constant over the entire thread length (24 in FIG. 1B) ofthe threaded connection, and a load lead 14 which is also substantiallyconstant over the thread length. The load lead 14 differs from the stablead 12 by a substantially constant amount referred to as the “wedgeratio.” The nominal lead 10 is the midpoint between the stab lead 12 andthe load lead 14.

A first example of a threaded connection according to the invention isshown in FIG. 3, wherein the stab lead 12 is increased from its originalvalue, beginning about two threads from the thread base of theconnection, to a higher value at the thread base end of the connection.In one specific embodiment, the stab lead 12 will be changed on both thepin and the box at the thread base ends of each of the pin and the box,however, it is to be clearly understood that the scope of the inventionincludes having the stab flank lead changed on only one of the pin orbox members according to the graph in FIG. 3.

In the embodiment of FIG. 3, the load lead 14 is decreased, starting atabout the same axial position along the thread length (24 in FIG. 1B) asthe change in the stab lead 12, namely about two threads from the threadbase, on the same member as the increase in the stab lead 12. In thisexample, the size of the change in the load lead 14 is substantially thesame as the size of the change in the stab lead 12. This results in asubstantially constant nominal lead 10 over the entire thread length ofthe particular member for the member on which the leads 12, 14 arechanged. Just as is the case for the change in the stab lead 12, thechange in the load lead 14 is preferably made near the thread base ofboth the pin and the box, but may be made on only one of the pin or boxmembers.

Another example of a threaded according to the invention is showngraphically in FIG. 4. In this embodiment, the stab lead 12 is decreasedstarting at about two threads from the thread base end of theconnection. The load lead 14 is increased correspondingly starting atabout the same axial position, so that the nominal lead 10 remainssubstantially constant over the entire threaded connection. As in theprevious embodiment, the change in stab and load leads may be made nearthe thread base of either or both the pin and box members.

Another embodiment is shown graphically in FIG. 5. This embodiment issimilar to the embodiment of FIG. 4, except that the load lead 14 andstab lead 12 are changed near the open thread end (first engaged thread)of the threaded connection. The embodiment of FIG. 5 also has asubstantially constant nominal lead 10 over the entire threadedconnection. As in the previous embodiments, the change in the stab 12and load 14 leads may be made near the open thread end of either or boththe pin and box members.

Another embodiment, shown graphically in FIG. 6, is similar to theembodiment shown in FIG. 3, the difference being that the stab lead 12and the load lead 14 are changed near the open thread end (first engagedthread) of the threaded connection. The embodiment of FIG. 6 also has asubstantially constant nominal lead 10 over the entire threadedconnection. As in the previous embodiments, the change in the stab andload leads may be made on either or both the pin and box members.

A different embodiment of the threaded connection is shown graphicallyin FIG. 7. This embodiment includes changes in stab lead 12 and loadlead 14 similar to those shown in both FIGS. 4 and 5. In the embodimentof FIG. 7, a change in the stab and load leads 12, 14 is made near bothends of the threaded connection. The embodiment of FIG. 7 also has asubstantially constant nominal lead 10 over the entire threadedconnection. As in the previous embodiments, the change in the stab andload leads may be made on either or both the pin and box members.

Yet another embodiment is shown graphically in FIG. 8. In thisembodiment, the stab lead 12 and the load lead 14 are changed near bothends of the threaded connection, in a manner which essentially combinesthe lead changes of the embodiments of FIGS. 3 and 6. This means theload 14 and stab 12 leads are changed at both ends of the connection.The embodiment of FIG. 8 also has a substantially constant nominal lead10 over the entire threaded connection. As in the previous embodiments,the change in the stab and load leads may be made on either or both thepin and box members.

A different type of threaded connection according to the invention isshown graphically in FIG. 9. In this embodiment, the stab lead 12 issubstantially constant over the entire threaded connection. The loadlead 14 is decreased near each end of the connection starting at abouttwo threads from each end. Notably, the nominal lead 10 changes when theload lead 14 is changed but the stab lead 12 is constant. As in theother embodiments described herein, the change in lead is preferablymade in corresponding (mating) positions on the threads of both the pinand box parts of the threaded connection, but may be made on only one ofthe pin or box members.

FIG. 10 shows a variation of the embodiment shown in FIG. 9. In thisembodiment, the load lead 14 is increased at about the same position ateach end of the threaded connection. The stab lead 12 is substantiallyconstant in this embodiment. As in the previous embodiments, the changein the stab 12 and load 14 leads may be made on either or both the pinand box members.

Another embodiment of a threaded connection according to the inventionis shown in FIG. 11. In this embodiment, the load lead 14 is increasednear the open end of the connection and is decreased at the thread baseend. The stab lead 12 is substantially constant over the entire threadedconnection. A variation of the embodiment of FIG. 11 is shown in FIG.12. In FIG. 12, the load lead 14 changes at the opposite ends of theconnection as compared with the embodiment of FIG. 11. As in theprevious embodiments, the change in the stab 12 and load 14 leads may bemade on either or both the pin and box members.

An embodiment shown in FIG. 13 is similar to the embodiment of FIG. 10,but in this embodiment, the stab lead 12 is increased at each end of theconnection, while the load lead 14 is substantially constant.Embodiments of the threaded connection according to the invention whichhave changed stab lead 12, corresponding to the changed load lead 14 ofthe embodiments shown in FIGS. 9, 11 and 12, are shown in FIGS. 14, 16and 15, respectively. As in the previous embodiments, the change in thestab and load leads may be made on either or both the pin and boxmembers.

In an embodiment shown in FIG. 17, the stab lead 12 and the load lead 14are both increased near each end of the threaded connection. Anembodiment shown in FIG. 18 has stab lead 12 and load lead 14 whichdecrease at each end of the threaded connection. As in the previousembodiments, the change in the stab and load leads may be made on eitheror both the pin and box members.

Another variation of the threaded connection is shown in embodimentsillustrated graphically in FIGS. 19 and 20. In these embodiments, thestab lead 12 and the load lead 14 are correspondingly increased near oneend of the threaded connection, and are correspondingly decreased at theother end. The embodiments of FIGS. 19 and 20 are similar, but have theincrease and decrease in the leads 12, 14 at opposite ends of thethreaded connection from each other. As in the other embodiments of theconnection according to the invention, the change in lead may be made oneither or both the pin and box members.

In all of the foregoing embodiments, the change in lead is shown asbeing about two threads from the associated thread end (open end orthread base) of the threaded member. This value has proven effective forthe thread diameter, taper and nominal leads used for the illustratedembodiments. This distance may also be defined as about twice thenominal lead of the threads in the part of the threaded connection wherethe leads are substantially constant. In other embodiments of a threadedconnection according to the invention, the actual axial position atwhich any lead change is started can be related to factors such as thethread diameter, the taper (rate of change in thread diameter) of thethreaded connection, and the type of thread used, among other factors.

The lead changes in the foregoing embodiments are all shown as beinglinear. Linear changes in lead are relatively easy to manufacture in anythreaded connection according to the invention, but it should beunderstood that some embodiments may include non-linear changes in leadwhile substantially achieving the benefits of the invention.

The foregoing description of a threaded connection was made with respectto a “dovetail” thread structure such as described in U.S. Pat. No. RE34,467 issued to Blose. It should be clearly understood that otherthread structures can be used in other embodiments of a threadedconnection according to the invention. One example of an alternativethread form suitable for use with a connection according to theinvention is shown in U.S. Pat. No. 6,254,146 B1 issued to Church. Thisthread is generally described as a “multi faceted” thread, and is shownin cross section in FIG. 21. The pin 2 includes threads having roots 48,crests 48A, load flanks 47 and stab flanks. The load flanks 47 of thethread includes facets 44, 45, 46 which form respective angles δ, Ω, εbetween the face of each facet and the centerline 40 of the pin 2.Corresponding facets 41, 42, 43 may be formed on the stab flanks 50 ofthe pin thread. The configuration of the flanks 47, 50 on the pin 2should be matchingly formed on the threads on the box 3. In the threadembodiment of FIG. 21, the threads on the pin 2 and box interlock, whilehaving a root 48 width that can be substantially the same as the crest48A width.

Another thread structure which may be used in various embodiments of theinvention is similar to a thread structure shown in U.S. Pat. No.4,600,224 issued to Blose. This thread structure is known as a “chevron”thread, and is shown in FIG. 22. The pin 2 and the box 3 may includestab flanks 53 shaped similarly to conventional “dovetail” threads. Theload flanks 51, 52 on the pin 2, and box 3, may include a “dovetail”portion 57, 56, respectively, and a chevron facet 55, 54, respectively.

The two alternative thread forms described above with respect to FIGS.21 and 22 are not meant to be an exhaustive representation of threadforms which may be used in various embodiments of a threaded connectionaccording to the invention. Other thread forms are described, forexample, in published PCT application WO 01/29475 A1 filed by Ramos etal. The thread forms shown in this publication include “squareshouldered” thread, which may have the flanks substantiallyperpendicular to the axis of the connection, or may include angledflanks.

Irrespective of the thread structure used in any embodiment of aconnection according to the invention, the common attribute of thethreaded connection is that at least part of the threads on theconnection include a stab lead which is different from the load lead,and either the stab lead or the load lead, on at least one of the femalemember (box) or the male member (pin) is changed beginning at a selecteddistance from an end of the threads.

Various embodiments of the invention can provide a threaded connectionwhich is stronger, after radial plastic expansion of conduit sectionsconnected thereby, than the threaded connections previously known in theart.

The foregoing description of a threaded connection according to variousembodiments of the invention was made in terms of a threaded connectionparticularly suitable for radially plastically expandable conduit. Ithas also been determined that a threaded connection made according tothe invention can improve the capacity of a threaded connection towithstand tensile stress and/or compressive stress along thelongitudinal axis of the conduit. Some embodiments may have increasedcapacity to withstand torque loading. Differential lead threadedconnections of the prior art are designed so that contact stressesbetween engaged threads are substantially evenly distributed, both alongthe helical length of the threads, and between the stab and load flanksupon “make up” of the threaded connection. The foregoing description ofvarious embodiments of the invention is made in terms of threadedconnections that can have more evenly distributed contact stresses afterradial plastic expansion of the threaded connection. As will be furtherexplained, embodiments of a threaded connection according to theinvention can also provide more even distribution of contact stressesunder compressive and/or tensile loads after make up of the threadedconnection. Other embodiments may have better capacity to withstandtorque loading. In this application of a threaded connection accordingto the invention, the connection does not have to be radiallyplastically expanded to obtain the benefit of the invention.

In order to more fully appreciate this particular application of athreaded connection according to the invention, it is helpful tounderstand certain characteristics of threads made according to variousembodiments of the invention. For example, referring once again to FIG.3 a threaded member which has its load lead and its stab lead convergingtoward the nominal lead (such as shown between engaged threads 8 to 10in FIG. 3) can be said to have thread which becomes “thicker” in thedirection of the lead convergence. Conversely, referring once again toFIG. 4, where the load lead 14 and stab lead 12 diverge toward one endof the connection, the thread can be said to become “thinner.”

Referring once again to FIG. 17, when both the load and stab leadsincrease, and in particular by like amounts, the thread can be said tobe “stretched out”, whereby spacing between threads increases (nominallead increases). Conversely, referring again to FIG. 18, as the load andstab leads decrease correspondingly, the thread can be said to be“pulling in.”

It has been determined that a threaded connection in which at least oneof the load lead and the stab lead, at least one end of either the malemember or the female member, is varied starting at a selected distancefrom the end of the thread, can have improved ability to withstandcompressive stress and tensile stress along the longitudinal axis of thethreaded connection, as well as, in some embodiments, resist fatigue andtorque.

Having explained the general concept of this application of theinvention, particularly advantageous embodiments related to thisapplication will now be explained.

In a conventional threaded connection, if the sum of the shear area(cross-sectional area of the thread root) is less than or equal to thesum of the thread flank area, such a connection is considered to be“shear weak.” In one embodiment of a threaded connection according tothis application of the invention, the thread is made thinner on boththe pin and box members near the first engaged thread. The thread isalso made thicker near the last engaged thread on both the pin (male)and box (female) members. FIG. 23 shows change in load lead 14 and instab lead 14 with respect to position along the thread length for thisembodiment. The embodiment of FIG. 23 may have better ability towithstand tensile loading than prior art threaded connections, becausethe shear stresses are more evenly distributed along the connection.

Another embodiment of a threaded connection according to the inventionis shown in FIG. 24. This embodiment includes increasing the stab lead12 and load lead 14 near the first engaged thread on both the pin memberand box member. This embodiment may have increased capacity to withstandcompressive loading.

Yet another embodiment of a threaded connection according to theinvention may have improved capacity to withstand torque loading andtension-induced fatigue (resulting from repeated application of tensilestress). This embodiment includes, on the male member, reducing the stablead near the last engaged thread (thread base of the pin). This isshown in FIG. 25A. On the female member, the load lead is increased nearthe last engaged thread (thread base of the box). This is showngraphically in FIG. 25B. This embodiment may have better capacity toresist cyclic compression/decompression induced fatigue.

In each of these embodiments, the change in thread lead which begins atthe selected distance from the end of the thread is shown as beinglinear. As in the other embodiments, the lead change can be other thanlinear. Furthermore, and also as previously explained, the selecteddistance may be some amount other than the two threads shown in theembodiments of FIGS. 23, 24, 25A and 25B.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A threaded connection for a radially, plastically expandable conduit,comprising: a male member having threads thereon defining a load flanklead, a nominal lead and a stab flank lead; a female member havingthreads thereon adapted to mate with the threads on the male member, thethreads on the female end defining a load flank lead, a nominal lead anda stab flank lead, the load flank lead and the stab flank lead on thefemale member and the male member changed at a predetermined ratebeginning at a selected distance from each end of the threads, the stabflank lead and the load flank lead being different from each other overat least part of a thread length, and wherein the change in lead islinear.
 2. The threaded connection as defined in claim 1 wherein theselected distance is about two threads.
 3. A threaded connection,comprising: a male member configured to mate with a female member, themale member comprising external threads corresponding to internalthreads of the female member; the external threads comprising a loadflank lead and a stab flank lead; the internal threads comprising a loadflank lead and a stab flank lead; wherein the load flank leads of theexternal and internal threads change at a first rate beginning aselected distance from both ends of the threaded connection; wherein thestab flank leads of the external and internal threads change at a secondrate beginning a selected distance from both ends of the threadedconnection, and wherein the first and second rates are linear ratechanges.
 4. The threaded connection of claim 3, wherein the first andsecond rates are the same.